1. Field of the Invention
This invention relates to the lubrication of a compressor for refrigerating machines, and more particularly to a compressor which is designed that the portions need to be lubricated of the compressor are lubricated by lubricating oil mixed in a circulating refrigerant, without providing an oil pan or oil pump.
2. Description of the Prior Art
A refrigeration circuit is generally composed of a compressor, a condenser, a liquid receptacle, an expansion valve, an evaporator and a piping interconnecting said elements, which are arranged such that a refrigerant is compressed by the compressor, cooled and liquified in the condenser, evaporated in the evaporator upon passing through the liquid receptacle and expansion valve, and then returned to the compressor. The compressor is usually provided with an oil reservoir at the lower portion of a crank case and lubricating oil retained in said oil reservoir is supplied to bearings, a piston and other sliding members as by a gear pump. With such a construction, however, the oil during the suction and compression stages of the compressor is discharged, although in a slight quantity, into the refrigerant passage through the gaps, for example, between the piston and cylinder wall, and finally the oil reservoir is emptied. Therefore, it is usual that the oil is separated from the refrigerant at the suction side of the compressor and returned to the oil reservoir. However, in such a construction, a high negative pressure suddenly appears in the passage on the suction side of the compressor at the start of said compressor, which acts in the oil reservoir via the gap between the piston and cylinder wall or the oil return passage, causing the so-called oil foaming phenomenon in which the lubricating oil bubbles in said oil reservoir. The oil foaming phenomenon causes the oil rapidly to flow into the passage on the suction side through the aforesaid gap or oil return passage, with the result that the oil reservoir is emptied and seizure of bearings or other sliding members results. Furthermore, when the negative pressure appears in the passage on the suction side, the pressure in the oil pan also drops, so that the performance of the gear pump is drastically degraded and the same trouble results. Such phenomenon is unavoidable in the type of compressor in which a fixed quantity of oil is stored in an oil reservoir and supplied to the sliding portions of the compressor by means of a gear pump or the like. This problem may be dealt with by making the oil reservoir larger and increasing the quantity of oil stored in said oil reservoir, but the larger size of oil reservoir makes the size of the compressor larger. This is undesirable particularly for car coolers and air conditioners which are subjected to a limitation in mounting space.